Metal product and method of making it



UNITED STATES PATENT OFFICE 2,051,454 METAL PRODUCT AND METHOD OF MARIN GIT Albert Wood Morris, Springfield, Mass.

No Drawing. Application September 8, 1934, Serial No. 743,308

7 Claims.

My invention relates to a new metal product and the method of making it. I will disclose the invention as a method for making a product made of sponge iron and indicate afterwards certain permissible variations.

In the preferred form of the invention I use sponge iron. This is ordinarily made by taking iron particles that have not been melted but occur in nature and can be obtained in powdered form. It is customary to make articles of sponge iron by molding the dry iron powder under mold pressure and afterward applying enough heat to the molded article to sinter the particles into coherent form in a non-oxidizing atmosphere. The above steps are known. The molded piece is characterized by the fact that it is lighter in weight per unit volume. It is quite porous even though the porosity may not be visible to the eye. This lighter weight porous iron metal, with no essentially difierent chemical composition than ordinary iron, is of great utility in the arts where its special attributes are neededfor example, a strong, tough, spongy iron that will absorb oil, or be of less weight at a particular part of a machine structure, or both.

So far as I know, this sponge iron has not been available in forms that are hardened while still retaining the sponge iron characteristics, which are so highly desirable. This will be appreciated from the fact that, if the iron is melted for alloying purposes to get it in condition for hardening by known nitriding methods, it will no longer be sponge iron. The full melting step destroys the sponge iron characteristics.

To get the sponge iron in hardened form (1. e., desirable parts 'iEf regions hardened while the remainder remains in unhardencd and machinable form) I proceed as follows: In originally molding the sponge iron, I face the mold surface or surface of the cores with a mixture of the sponge iron powder and aluminum powder at the spots where the corresponding parts of the product to be molded are desired in hardened form. For example, if I am making a sponge iron cube and desire the corners hardened, I first pack the corners or the mold with a thorough mixture of sponge iron and aluminum powder (in about the proportion of flfty of iron to one of aluminum), and to the depth and area desired.

when that is done the mold is filled with sponge iron alone. Next, the article in powdered form is compressed by the mold parts moving inwardly. Heat is applied after the powdered article is taken out of the mold to sinter all the particles together in the ordinary sponge iron manufacturing way. When the article is finished in its sintered form, it has all the sponge iron characteristics. The mixture of the aluminum has resulted in a mixture and not an alloy 5 of the metals at the chosen regions.

Then this sponge iron article is taken and heated in a closed retort, for example to 800- 1100 degrees F. While heated to such a temperature, ammonium gas is admitted to contact 10 the sponge-like article in the retort. The aluminum powder mixed in the sponge iron article helps dissociate the nitrogen in the gas. Because the iron form is porous the gas will penetrate. At the regions where the aluminum powder is 15 mixed with the sintered iron, the nitrogen gas is liberated. It is in nascent condition. It has the aluminum to help it nitride the iron, which it will do. It nitrides the iron only where the aluminum has been located in the mixture of 20 powdered metals. It hardens those regions without having any appreciable effect on the other regions. This operation of nitriding is a very short and inexpensive one compared to nitriding metals in any other way. And with respect 25 to sponge iron (on account of its not being subject to an alloying step in preparation for the nitriding step), I know of no other way to nitride. Of course sponge iron in contact with nitrogen over a long enough period will get a small degree 30 of nitridization, but not suflicientto be practical for my purpose. In my method the nitridization of the sponge iron is located in the chosen region, for the chosen depth, and is rapid enough to get the eifect without losing the desired 35 sponge iron characteristics in the other regions.

As I have stated, my invention is primarily directed to the advantages of making a sponge iron article. But my nitriding method, while directed to that object as a chief purpose in prodiicing a particular article, also has an advantage in nitriding other articles.

In this connection I can powder any nitridible metal (iron alloys), mix it with other powdered metals, and make a sintered product in which portions (of the nitridible metal) are hardened. But I have found that while an article may be made of ordinary iron powdered, pressed to form, and with the particles sintered together as I have described in connection with sponge iron, it is not so good. It can be nitrided in chosen regions while the others are left soft for machining purposes. It will be seen under the microscope and by other tests that ordinary iron so treated will not give anywhere near as 65 good an article as sponge iron. It seems that the powder of ordinary iron as contrasted with the particles of iron obtained in their natural state, as used for sponge iron, does not have the same physical characteristics for making as good an article by the sintering method. The ordinary iron article is not so porous and not so strong as the sponge iron when made by the sintering method from powdered form. But in some uses it may be desirable to have such an article of ordinary iron made with hardened spots, and my new nitriding method is claimed broadly in that respect, as well as more narrowly with respect to sponge iron.

I believe, on account of the physical characteristics of the sponge iron, my nitriding method has an especially eflective action in hardening that is superior, as compared to its effect with ordinary iron.

I do not wish to limit my invention to the use of aluminum, as other metals, such as zirconium and chromium, are known as having equivalent afiinity for nitrogen. They will also free the nitrogen from the ammonium gas for nitriding purposes. I may use aluminum and iron powder in relative proportions of one to fifty, as above stated. There is no exact proportion necessary, but it will be understood that the iron powder far exceeds the aluminum, with the object of retaining iron characteristics in the hardened regions.

So far as I know I am the first to nitride iron by powdering it, mixing it with another powdered metal like aluminum, forming an iron article, sintering it for cohesion, and then subjecting the porous article to heat, and surrounding it with an ammonium gas atmosphere for the regional nitriding action. But, in addition to the broad features of such an invention, I desire to claim as a specific invention the discovery of making sponge iron articles with regional nitrided spots. The specific article per se has great utility, and no one, so far as I am aware, has ever before made it.

Having disclosed my invention, I claim:

1. The method of making a sponge iron product with a hardened portion which consists in shaping the product in dry powdered form and in this step locating the hardened portion by mixing with the iron at such portion a small proportion of powdered metal having great affinity for nitrogen, subjecting the product thus formed to a sintering operation to get the powdered metal into coherent spongy formation under the sintering heat, heating the latter formation in a range slightly below the melting point of any metal in the mixture, and in an atmosphere of ammonium gas until the predeconsists in shaping the product in dry powdered form and in this step locating the hardened portion by mixing with the iron at such portion a small proportion of powdered metal having great aflinity for nitrogen, subjecting the product thus formed to a sintering operation to get the powdered metal into coherent formation under the sintering heat and without melting the whole mass, heating the sintered formation to a range slightly below the melting point of any metal in the mixture, and in an atmosphere of ammonium until the predetermined portion is nitrided.

3. A metallic product consisting in a spongy mass of sintered virgin iron particles having a part impregnated with a nitride-promoting metal, such impregnated part of said mass being nitrided for hardness.

4. A metallic product consisting of sponge iron having one or more spots impregnated with a. nitride-promoting metal and nitrided to a substantial depth and unmachinable in the hardened sponge iron form, and the rest of the product being of machinable sponge iron characteristics.

5. A ferrous metallic product of porous formation of sintered metal having one or more spots of substantial depth composed of mixed metals, one of iron and one of a metal having great aflinity for nitrogen, the latter being in small proportion to the ferrous .metal, the ferrous metal in said spot of mixedmetal being nitrided.

6. The method of making a sponge iron product with a hardened portion which consists in shaping the product in dry powdered form and in this step locating the hardened portion by mixing with the iron at such portion a small proportion of powdered aluminum, subjecting the product thus formed to a sintering operation to get the powdered aluminum into coherent spongy formation under the sintering heat, heating the latter formation in a range slightly below the melting point of both iron and aluminum and in an atmosphere of ammonium gas until the predetermined portion is nitrided, whereby the product has sponge iron characteristics with a hardened portion.

7. The method of making a sintered iron metal product with a hardened portion which consists in shaping the product in dry powdered form and in this step locating the hardened portion by mixing with the iron at such portion a small proportion of powdered aluminum, subjecting the product thus formed to a sintering operation to get the powdered metal into coherent formation under the sintering heat and without melting the whole mass, heating the sintered formation between 800 and 1000 degrees F. in an atmosphere of ammonium until the predetermined portion is nitrided.

ALBERT WOOD MORRIS. 

